In an LED driver which uses feedback, a signal representing the voltage at the output or the current flowing to the output is fed back to enable control of the power converter.
The power converter typically comprises an inductive switch mode power supply. The current flowing through an inductive element is controlled by a main converter switch, and the duty cycle with which the switch is controlled determines the power conversion ratio.
It is required to prevent that the inductive element becomes saturated, since the current can then run out of control. In normal operation, the feedback mechanism prevents this saturation. However, there may be a delay before the feedback mechanism is effective, so that at start-up of the power converter, the currents flowing may be out of control. For example, if there is a short circuit condition at the output when the power converter is initially turned on, excessive currents may flow before the feedback mechanism is able to regulate the system.
In order to address this problem, the inductive element is typically scaled up to prevent it becoming saturated.
An alternative is to implement a soft start circuit to limit the initial current through an inductive element. However, this generally requires significant additional control circuitry and is therefore expensive to implement.
U.S. Pat. No. 7,333,353 discloses a self-oscillating flyback converter with a soft start function. The soft start function influences the initial ramping up of current through the main converter switch.